In the midst of declining fossil fuels reserves and a great expansion of natural gas production, increasing effort is seeking to commercialize the conversion of methane into chemical feedstocks and fuels as an alternative to petroleum. Large natural gas reservoirs exist throughout the world and, thus, have an enormous potential as a clean fuel and chemical feedstock. Currently, the vast majority of natural gas is used for heating purposes. This is due largely to the properties of methane as a heating fuel as well as the difficulty in economically converting methane into larger, higher value chemicals and liquid fuels.
Several methods to convert methane indirectly and directly into higher olefins have been report in literature. However, the two large-scale methods being used commercially are methanol-to-olefins (MTO) and the Fischer-Tropsch synthesis (FT) (Alvarez-Galvan et al., Catalysis Today 171, 15-23, 2011). Both these processes first involve the generation of synthesis gas (Syngas), a mixture of H2 and CO (Alvarez-Galvan, supra, 2011). Syngas production is energy intense, requiring high temperatures (>600° C.), making it a costly process that typically represents 60% of total capital costs (Aasberg-Petersen et al., available at: www.topsoe.com/business_areas/methanol/˜/media/PDF %20files/Methanol/Topsoe_large_scale_methanol_prod_paper.ashx, 2008). Thus, direct routes for conversion of methane to olefins and other chemicals are conceptually preferable and many methods have been reported (Alvarez-Galvan et al., supra, 2011). However, these reported methods suffer from low yield and low product specificity rendering them uneconomical thus far. Therefore, a need exists for a novel, more cost effective route to produce chemical feedstocks and fuels from methane.
The challenges in the overall conversion of methane to chemicals, either directly or indirectly, largely stem from the activation energy required to convert methane into large molecules. This is a result of stability and symmetry of methane. Current solutions to activate methane involve the use of inorganic catalysts such as palladium, which serve to reduce the energy required for methane activation (Aasberg-Petersen et al., supra, 2008). The use of high temperatures and pressures are also needed. These processes are extremely energy intense and also lack product specificity, requiring additional purification steps to separate the various products.
To circumvent these pitfalls, described herein are biosynthetic pathways for the conversion of ethylene, which may be converted from methane with established methods, to acetyl-CoA. The biological assimilation of ethylene has only been reported in methanotrophs (Bull et al., Nature 405, 175-178, 2000; and Treude et al., Appl Environ Microbiol 73, 2271-2283, 2007). Due to the difficulties in culturing methanotrophs and very few genetic modification tools, no large-scale applications of these organisms has been demonstrated. Therefore, the present disclosure describes the construction of ethylene assimilation pathway in recombinant bacteria, which already have a plethora of available genetic tools and has well-established large-scale applications. Furthermore, several examples of pathways for converting basic metabolites to fuels and chemicals (e.g., acetyl-CoA to n-butanol) in E. coli have been extensively reported (Rabinovitch-Deere et al., Chemical reviews 113, 4611-32, 2013). Thus, by engineering a high flux ethylene assimilation pathway in recombinant bacteria, better performance may be achieved than what has been demonstrated in methanotrophs.
Lastly, since ethylene is already a high volume chemical feedstock used in the chemical industry, a high performance ethylene assimilation pathway in recombinant bacteria could enable immediate industrial applications. Due to the broad uses of ethylene as a chemical feedstock, technological innovations for the conversion of methane to ethylene will be developed. Therefore, a well-engineered ethylene assimilation pathway in a user-friendly host such as recombinant bacteria will enable the biological conversion of ethylene into liquid fuels and other high value chemicals.